The subject invention relates to solder pastes or creams and more particularly to the vehicles used therein.
Today's electronics industry requires the production of circuit boards that are more complex and more densely arranged then those used in the past. Accordingly, there is need for solder pastes and creams capable of producing smaller, more closely spaced solder joints. Currently, solder paste vehicle are limited in producing solder pastes capable of achieving ultra small joints with high density. This has retarded the growth of the electronics industry and has limited the development of new technologies.
Solder pastes and creams (hereinafter, "pastes") have been used for years in the microelectronic industry for forming strong mechanical joints and electrical connections between discrete electronic components and conductive pathways. These pastes, as with all solder materials, require a chemical flux to facilitate the soldering action. This flux is used to prepare the surface to be soldered and the solder metal, by removing the surface oxides present on both surfaces. This chemical cleansing action allows molten solder to form a metal-metal bond with the surface to be soldered, essential for good electrical and mechanical bonding.
Many types of fluxes have been used in soldering in general, and in solder pastes and creams in particular. The military specification QQS571 described many of the requirements that these materials should possess. In general there are four major categories of fluxes based on composition and fluxing strenght; R, RMA, RA and OA. The R, RA and RMA grades materials are based on using the chemical properties of wood rosin and chemically modified resins to provide the fluxing action. The R grade is the weakest of the three types, the RA the strongest. Differences in fluxing action are obtained by using rosin/resins with more acidic content and by incorporating other non-resin chemicals into the formulation. These chemicals can include various carboxylic acids, chlorinated hydrocarbons and amine activators. The OA grade does not use any rosin or resin, and is based on the use of organic acids, mostly carboxylic acids, to provide the fluxing action. In addition to the above-described chemically active constituents, fluxing formulations used in solder cream and paste manufacture include two other major classes of materials: binder and solvents.
Binders provide viscosity and other thixotropic properties to the vehicle. In addition to solid binder, solid or liquid surface active agents can be incorporated to impart proper wetting, flow and storage properties to the vehicle.
Solvents act primarily to dissolve fluxes, activators, binders and surface active agents. A solvent can be a single solvent or a mixture of different types of solvents. Solvent liquids are used for their dissolving ability and to impart certain theological (flow) properties to the paste.
Solder pastes or creams are made by creating an intimate mixture of solder powder and gel-like vehicles. The powders are finely divided mostly spherical, particles. They can have various chemical compositions depending on the makeup of the metal alloy. The size and shape of these powders play an important role in determining which vehicle system can and should be used to produce good mechanical solder joints. For many years, solder powders with a size distribution falling within the limits of a 200 and 325 standard ASTM sieve were considered adequate for most microelectronic soldering applications. Most fluxes, whether weakly activated R grade or highly activated RA grade, used these types of powders. With the increasing sophistication and component density requirements of printed circuit boards, the standard powders used in conjunction with existing vehicle technology were not able to produce solder with the necessary density.
There was and is a growing need for finer powders which yield smaller, more closely spaced solder joints. This creates a greater demand on the vehicle systems used in these solder pastes. As the solder powder decreases in size, for example, from 400 to even 500 mesh powders, the surface area of the powder increases. This in turn increases the amount of surface oxides. Standard R or RMA type vehicles may not be sufficiently activated to facilitate proper fluxing action. In addition, the use of these finer powders changes the thixotropic behavior of the materials. The shear thinning properties are similar but not necessarily the same as when coarser powders are used.
Many of the chemicals used in the above-described formulations are not fugitive in the temperature range that is used to melt (reflow) the solder powder component of the paste. Therefore, upon cooling to room temperature, there remains an organic residue surrounding the soldered joint, made from some of the chemicals used in the vehicle system. In general, the majority of solvents have evaporated leaving a residue that is typically viscous or hard.
The microelectronics industry has traditionally removed residues in a post soldering cleaning operation. The purpose is two-fold. First, the finished circuitry is aesthetically more acceptable, and second, once the residues are removed, there exists less propensity for the circuitry and solder joints to be adversely affected by degradation brought on by ionic contamination.
The residues have been traditionally removed by chlorinated solvents and alcohols. These solvents are able t dissolve the remaining organic residue and have a low boiling point and high vapor pressure to facilitate fast evaporation and efficient drying and removal of residual solvent. The main type of solvents use for this purpose are so-called CFC's, chloro-fluorohydrocarbons.
The environmental danger, e.g. to the ozone, caused by CFC solvents, together with legislation, has motivated the industry to experiment with laternative cleaning solvents and vehicles. There now exist organic acid vehicles which are water soluble and are cleanable with just water or water in conjunction with a siphoning agent (essentially, soap and water). In addition, solvents with less ozone depleting power have been developed to replace CFCs, e.g. tepene and HCFC solvents.
Because of the cosmetic and reliability concerns described above, residues must meet a number of stringent criteria. First, the vehicle residue should be below a certain ionic threshold value. Accordingly, the ionic activity of the flowing vehicle must be relatively low so that the ionic strenght of the residue is also low. On the other hand it is important to maintain the ability to produce dense fine space solder joints, i.e. by using finer solder powder.
The second important concern is reducing the flow of the residue away from the solder joint. This property is referred to as "hot-slump". After solder reflow, residue is typically left around the perimeter of the solder joint. It is important that the remaining residue be hard and not tacky to reduce the possibility of contamination by dust and dirt particles which may cause electric shorts or degradation of the solder joint or other board circuitry. Therefore, a major requirement for a no-clean vehicle systems is a hard residue with low flow properties at elevated temperatures.
Many different ingredients and mixtures have been suggested and used in an attempt to prevent "hot-slump". For example, prior art attempts have typically included the use of high viscosity compounds such as petroleums and gum thickeners. However, the pastes of the prior art were limited because they spread upon heating, were unformable into small dots, were unstable (i.e. prematurely coagulated),were too viscous to dispense, and left residue which required toxic chemicals to clean.
The subject invention fulfills a long flex need for a no-clean vehicle having excellent "hot-slump" properties and a capacity to permit the soldering of closely spaced joints.